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Topic: Junkers CLM (Read 1862 times)

A Junkers pattern opposed piston 2 stoke diesel has been on my build list for some time. I now think that I am close enough to getting my horizontal diesel to run to start on the design for this.The prototype is the smallest of these engines with 65mm bore and 90 + 120mm stroke which was used in tractors and boats and was manufactured under licence by CLM in France.

There are a number of interesting design features:

The unequal strokes are supposed to allow better port timing. This will require some simulations to reach a suitable design.

The scavenge pump is mounted on top of the top piston and is square. I think this will have to be redesigned round even if this reduces the capacity.

The top of the liner is unsupported and the ports open directly into the crankcase.

The top bearings on the pullrods are ball races, I assume due to borderline lubrication

I am basing my design on a 25mm bore which gives strokes of 35 + 46mm. This will give a liner around 135mm long which I think I can bore using an idea from Jason B of an asymmetric boring bar mounted between centers and the liner clamped to the saddle. The crankshaft also appears possible. Checking on the lathe the clearance over the cross slide is 33.5mm rather than the 31mm in the manual. That extra 2.5mm will allow me to use a chucking piece supported by the tailstock.I do also have the option of reducing the bore to 20mm but from the diesel point of view the larger the capacity the better.

I am planning to fabricate the cylinder block and crankcase from steel rather than carve it from aluminium so I need a bit of welding practice first.

A modern version was built by Eco Motors. As far as I know, they're not in business anymore. They had a large version with an electrically boosted turbocharger and a small version with piston pumps. See the attached files for the small version..

Thank you for the video Niels - I wasn't aware of an engine museum in Rødvig, but after my dad moved to Nødebo. I sometimes take the ferry from Rødvig to visit him, so I will definitely visit it in the future.

I do not see any transfer port in the previous drawings - is that through the upper scavenge piston down towards the upper piston and out the sides of it into some short transfers ?

As I said elsewhere, you're in for a challenge here Roger - one I very much would like to follow and hopefully see run in the end.

Thank you for the video Niels - I wasn't aware of an engine museum in Rødvig, but after my dad moved to Nødebo. I sometimes take the ferry from Rødvig to visit him, so I will definitely visit it in the future.

Thank you for the video Niels I have a short clip from somewhere else of the coin balancing on the same engine. The top piston and pull rods must be heavier than the bottom piston but the stroke is less Has someone done something clever

I have given this a bit more thought. The basic dimensions will be taken fron my horizontal diesel, the short connecting rod will be identical but the long pull rods are in tension so all the firing load is on the big end bolts. I did some stress calculations for the horizontal engine which gave a good safety margin with M3 bolts but I think these will have to be increased to M4. As the stroke is shorter the additional width of the bearing won't require a bigger crankcase. As the rods are in tension buckling won't be a problem so I think they can be reduced to 8mm diameter. The big ends will be reduced to 10mm wide to keep the crankshaft as short as possible.I had a dig through my material stocks and have a cylinder block, cylinder liner, crankshaft and pistons.

Roger,Here is another OP diesel engine design from Achates Power currently in a joint research project with Cummins Engine Co. for development of combat vehicle engines for the US Army. It is a compact engine following more contemporary engine package design features. It does use a blower for scavenging which might complicate a smaller model engine design. Watch their video.Jeff

Thank you Jeff There seem to have been many attempts at opposed piston engines but they have only been successful in larger sizes

I am slowly working through the design. I now understand the square scavenge piston It is so the ends of the pull rods for the top piston can go into the scavenge cylinder and reduce the height of the engine (and the piston).

The next step is to work out a governor design. It is mounted on a vertical shaft in the front part of the crankcase driven by a pair of helical gears. The principle is similar to the one I was experimenting with for the horizontal diesel:

In this case the bottom plate is mounted on a bearing and the balls are driven by a spider keyed to the shaft. A PDF of the principle is attached. I think I will have to make a trial version. The shaft is 6mm diameter and the overall diameter is 32mm with 6 6mm diameter balls.

As I continue with the drawings the governor is a key part of the front of the crankcase so time for experiments

I decided to make the drive spider from Delrin and the thrust plate and cone will be steel (case hardened if it works ) The basic form was turned and the keyway cut. As it was Delrin it just needed finger pressure on the broach The blank was mounted on a short length of 6mm rod with a key slot and put in my Proxxon RT. I cut the slots with a 6mm 3 flute end mill in two steps.

A quick check with some 6mm ball bearings and onto the cone. This and the thrust plate were turned from a length of 35mm free cutting steel. The thrust plate was polished before parting off. Next the cone was reversed in the 3 jaw chuck using a couple of parallels (6mm HSS tool blanks) to keep it in line with the face of the chuck.

The cone was then drilled 9.8mm and reamed 10mm. The cone itself was turned using my little top slide. I used the body of a square to align the thrust plate at the front of the chuck so it could be faced and then moved it back for drilling and boring.

That’s an ingenious, compact design, Roger. It will be interesting to see how it performs, and how much force it can produce on the levers. I assume the top cone also spins so the balls only move in and out?

In this version the top cone is fixed so it is it's own thrust bearing. The thrust plate will rotate at twice the shaft speed. I tried a couple of other variants in my diesel thread. This design is the same as was used on the full size version of the engine.

The bronze bush was turned, pressed in place and reamed again. The sleeve was made from 8mm silver steel, hardened and polished. I made a trial shaft from 6mm silver steel and cut a 2mm keyway to drive the spider.

I drilled and tapped an M3 hole in the side of the cone to be able to apply a load and assembled the system for a trial in the bench drill (it has a speed indicator). With the two springs I was using it would lift between 2000 and 2500 rpm depending on the preload (set by winding the table up and down).